It is understood the aircraft went down southeast of Okinawa at approximately 2:45pm local time on Wednesday. The US Navy says the names of those involved are being withheld until their family and next of kin can be notified. 

The aircraft was en route to the US Navy aircraft carrier USS Ronald Reagan (CVN 76), which is currently operating in the Philippine Sea. The cause of the crash is not yet known.

The crash is the latest in a series of series US Navy incidents this year. Less than a week ago, a destroyer collided with a Japanese tugboat in Sagami Bay, off the east coast of Japan. In June, seven soldiers were killed when the USS Fitzgerald collided with a Philippines container ship in the middle of the night southwest of Yokosuka. The USS John S. McCain crashed into a commercial vessel in the Strait of Malacca off the coast of Singapore in August. Ten sailors drowned as seawater filled sealed compartments of the ship. In all, there have been six such incidents since the start of 2017.

https://www.rt.com/usa/410585-navy-aircraft-crash-pacific/